It is a well known that various forms of asbestos, a mineral fiber, have been used for many years as diaphragm materials in various aqueous systems, especially chlor-alkali electrolytic cells. Ordinarily the diaphragms have been prepared by vacuum-drawing a slurry of asbestos fibers onto a porous cathode, thereby depositing a mat of asbestos on the cathode.
In recent years various polymer-bonded asbestos sheets and polymer-containing diaphragms have been prepared as shown by, e.g., U.S. Pat Nos. 3,017,338; 3,097,990; 3,153,610; 3,551,205; 3,583,891; 3,694,281; 3,704,221; 3,723,264; 3,505,200; 3,723,264; 3,853,720; 4,093,533; and 4,142,951. Other variations from the basic asbestos art are found in U.S. Pat. Nos. 4,036,729; 4,070,257; 4,081,350; and 4,126,535.
It is becoming increasingly desirable to find alternatives to the use of asbestos and this extends to uses in chemical processes, e.g., chlor-alkali electrolytic cells, especially diaphragm cells.
It has now been found that a suitable alternative to asbestos has been found in the use of pulverized expanded vermiculite as a diaphragm material in electrolytic cells. Whereas such pulverized expanded vermiculite structures are useful in other applications, this disclosure is particularly directed to such structures suitable for use in chlor-alkali electrolytic cells.
It has also been found that, in the making of diaphragms or membranes for chlor-alkali electrolytic cells, the expanded vermiculite requires additional treatment that is not required of asbestos, that is, the expanded vermiculite needs to be ground up (pulverized) to cause substantial separation of the platelets which comprise the expanded bundles; such pulverization causes a wide range of particle sizes which has been found to be a beneficial feature.